Which of the following can determine that the M2A1 barrel is hot?

Determining that the M2A1 barrel is hot is critical for safe operation, and this can be accurately identified by supervision, specifically when a Team Leader or Vehicle Commander assesses the barrel temperature as hot. This evaluation is important because it is based on the actual conditions experienced during operation, including the number of rounds fired and the heat generated as a result.

In this context, the role of the Team Leader or Vehicle Commander is pivotal as they are trained to monitor such conditions and ensure the safety of personnel and equipment. Their judgment encompasses not just the number of rounds fired but also considerations about the weapon's performance and operational safety. While factors like the number of rounds can indicate a potential for overheating, the explicit acknowledgment from qualified personnel that the barrel is hot provides a clear directive to proceed with caution.

The options about deeming the barrel cold or performing an environmental assessment do not directly pertain to the immediate assessment of the barrel's temperature. Environmental factors and arbitrary round count assessments contribute to the overall situational awareness but are secondary to the immediate and authoritative judgment of trained personnel on the weapon's status.